1. Field of the Invention
This invention relates to a shift control system of an automatic transmission, and more particularly to improvements in a shift control system of an automatic transmission, wherein the shift control system includes at least a first and a second transmissions capable of automatically switching speeds separately of one another, and the first and the second transmissions are shifted simultaneously or alternately, to thereby achieve multi-speed shifts.
2. Description of the Prior Art
Along with a rapid spread in use of the automatic transmissions for motor vehicles in recent years, there have been commonly adopted such transmissions wherein a so-called over-drive device, in which a transmission gear ratio is less than 1, is connected in series to the first transmission capable of automatically switching the shift speeds in association with a vehicle speed, a throttle opening, etc. as the second transmission.
Furthermore, there is also known such a transmission wherein, based on a function of the second transmission capable of switching from lower speed to higher speed as the above-described over-drive device, shift controls shown in FIG. 2A for example are performed, so that multi-speed shifts of 6 forward speeds can be achieved. This transmission is of such an arrangement that a shift of the second transmission is actively cooperated with a shift of the first transmission, whereby the first transmission and the second transmission are shifted simultaneously or alternately, so that multi-speed shifts can be achieved.
The above-described arrangement makes it possible that the existing automatic transmission is utilized as the basis, and changes in design are minimized for manufacturing advantage, so that multi-speed shifts can be achieved. As the result, such advantages can be offered that the fuel consumption rate is improved, the power performance is bettered, the burden of frictional materials is relieved due to making the speed shifts into multi-speed shifts, and the like.
However, in the automatic transmission wherein the first and the second transmissions are shifted simultaneously or alternately to achieve the multi-speed shifts, as shown in FIG. 2, there occurs a case where the first transmission is high gear shifted and the second transmission is low gear shifted, for example, like a shift from a second speed to a third speed and like a shift from a fourth speed to a fifth speed, to thereby up shift the automatic transmission as a whole. At this time, if only the respective shifts are controlled separately of one another, an increase in shift shock is not avoidable. Furthermore, for example, while an up shift is in operation, the shift is started from a down shift, or a down shift after an up shift is performed, thus presenting such a disadvantage that there may be experienced the shift characteristics of a strange driving feeling.
On the other hand, if the release of an acting force on the side of the second transmission is too rapidly performed, then there may be presented such a disadvantage that a neutral portion occurs briefly in an intermediate portion of a series of torque transmitting systems. If such conditions occur, then, when the second transmission is disposed on the input side of the first transmission for example, an engine revolution speed is rapidly increased commensurate to a throttle opening and rotary members of the first transmission are rapidly accelerated because of no load. However, the torque of an output shaft of the first transmission (an output shaft of the automatic transmission) is rapidly lowered and the shift shock is increased accordingly. And, when the second transmission is disposed on the output side of the first transmission, the second transmission is out of the torque transmitting systems, whereby the torque of an output shaft of the second transmission (an output shaft of the automatic transmission) is lowered similarly to the above, and the shift shock is increased as well.
As a method to cope up with the above-described disadvantages, it may be contrived that an acting force of a frictionally engaging device of the second transmission during the operation of shift is maintained on a predetermined level for a predetermined period of time. However, in this case, if the predetermined level is uniformly set, there occur possibilities that a shift is not started, or the lowering effect of the shift shock is not obtained depending on the throttle opening or the magnitude of the engine torque at that time.